In a typical example of an actuator drive designed to be used to operate a motorized valve, the driving member is an electric motor and the reduction drive to the driven shaft is in the form of a worm and worm wheel, the latter being keyed or otherwise fixed to the actuator output shaft or “column” which operates the moving element of the valve.
In valve actuation technology it is common practice to arrange the drive system such that only forces and torsion generated by the driving rotating member or members of the actuator can be transmitted through the gearing to the driven rotating member this is to eliminate the problem that can arise when no power is being provided to the driving member and the unbalanced forces generated by the pressurised fluid within the valve are sufficient, when acting on the valve moving member, to “back drive” the actuator gear train resulting in the valve moving member being displaced from its seating or from its last position in the fluid path determined by the last operation of the valve actuator.
One common method used to eliminate- the back driving possibility in an actuator gear train is to use a substantial reduction ratio on the worm/worm wheel mesh, the resulting low value of the worm lead angle combined with the normal friction coefficient at the meshing surfaces causing the drive to lock when torque is applied to the worm wheel side of the drive.
In a motorized valve this “locked in” torque remaining after a powered cycle operation can be considerable, particularly in valves where the moving member is forced on to a seat in order to eliminate fluid leakage between the upstream and downstream valve ports.
In order to be able to operate the valve in the event of a power failure or when installing or servicing the pipe line equipment, a two position clutch and a hand operated wheel are provided. These members are normally provided on the actuator output shaft which carries the worm wheel. The clutch is operated by a manual lever. The lever is spring biased to a parked position and the normal state of the drive mechanism is to be clutched into the power driving mode. Operating the lever over its full arc of travel uncouples the clutch moving member from the power driving position and moves it into the manual driving position. Simultaneously, the lever operates a latching mechanism which ensures that the clutch remains in the manual driving mode when the lever is released and returns to its parked position. The latching mechanism is so designed that it will hold the actuator in the hand drive state until the power drive is energised again. Rotation of the de-clutched member of the drive train then automatically releases the latch mechanism and allows the clutch moving member to return, under spring load, to its power driving position.
This automatic reversion to the power driving mode is a common feature of Valve Operation Technology and has come about to avoid the need to send operating staff out, sometimes long distances, to un-manned sites to attend to valve actuators which have been inadvertently left in manual drive mode after a service visit etc.
The high torque generated in the actuator output shaft and remaining locked into the shaft by the non-back-driving worm/worm wheel mesh generates high forces on the clutch engaging members when the said clutch members are mounted on the output shaft and so become part of the force/torsion path between the seated valve and the locked meshing teeth on the worm wheel. These relatively high torques and forces can cause difficulties to arise when trying to disengage the clutch from the power drive position.
Further, on large bore valves, where considerable forces need to be used to both seat and un-seat the valve, the output shaft mounted hand wheel as well as the lever used to uncouple the clutch from the power mode and move into hand drive mode need to be of substantial, all metal construction, adding considerably to the size and weight of the actuator.
A partial solution to this problem is revealed in U.S. Pat. No. 4,370,902 in which an additional spur gear reduction train is inserted between the worm wheel shaft and the actuator output shaft or column. This reduces the torsion on the worm wheel shaft approximately in proportion to the respective teeth numbers on the additional meshing spur gears but still leaves the torque from a seated valve spindle reduced by the gear drive ratio locked into the power drive side of the actuator clutch.
The present invention sets out to eliminate the locked in torque from the power driving side of the clutch assembly and to further reduce the operating torque required at the hand wheel so that, in addition to requiring less manual effort to operate the lever and hand wheel, the components that make up the clutch, lever and hand wheel assemblies can be reduced in size and, if required, can be manufactured from moldings made of plastic or reinforced plastic materials.
A typical existing method, used in Valve Actuation Technology to hold the actuator drive in hand mode after operating the lever, but automatically to change to power drive as soon as the drive motor starts to rotate, is achieved by providing a propping mechanism between one face of the worm wheel and the clutch moving member. This mechanism holds the clutch moving member in its hand drive position and compresses the spring means which urges the clutch moving member into its power driving position. The essential feature of the prop mechanism is a pivoted latch member which forms the end of the prop adjacent to the worm wheel. The latch member is provided with a light centering spring which holds the member at right angles to the worm wheel surface when the hand operation mode has been selected.
As soon as the worm wheel starts to rotate under power drive the friction between the wheel moving surface and the foot of the latch member overcomes the light centering spring means and causes the latch member to rotate approximately 90 degrees to a trailing, parallel position on the worm wheel surface. The resulting movement of the latch member pivot, in an axial direction relative to the worm wheel shaft is sufficient to allow the clutch moving member to engage power drive.
Whilst this existing automatic re-engaging to power drive arrangement is satisfactory, it does rely on the use of small wire centering springs which may fail in service.
Also the need to rely on surface friction between the latch member and the worm wheel surface in order to operate the mechanism results in the latch not being “positively locked” into the hand driving mode.
It is a further object of this invention to provide a latching mechanism for the automatic clutch re-engagement operation which does not rely on friction forces to release the latch and further, to provide a mechanism in which the latching member is not in contact with the rotating members of the drive when the clutch is in the power drive mode.